Device for zone melting



June 8, 1965 M. BRUNET ETAL DEVICE FOR ZONE MELTING Filed Dec. 17, 1962 MICHEL I EMILE DE YRIS .AGEALI' United States Patent 3,188,373 DEVICE FOR ZONE MELTING Michel Brunet and Emile Deyris, Caen, France, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Dec. 17, 1962, Ser. No. 2.453% Claims priority, application France, Dec. 15, 1961,

9 Claims. c|. 13-1 This invention relates to a device for zonemelting comprising a tube intended for an elongated charge of a material to be treated by zone melting and at least one member for locally heating the charge, together with means for displacing [the tube in the direction of its length relative to the heating member. The term displacing the tube relative to the heating member is to be regarded herein in a wide sense and also includes the use of a stationary tube and a displacement of the heating member.

In such a known device, the tube is journalled on each side of the heating member. The heating member comprises a heating ring of graphite and a high-frequency coil surrounding it. The heating ring is so positioned that the tube does not contact with it. The two bearings for the tube are located at a comparatively large distance from the heating member.

For carrying out zone-melting processes satisfactorily, the length of the molten zone and the shape of the coagulation front must preferably remain substantially con.- stant as the molten zone is moving through the charge and, if repeated, be reproducible. To this end, it is necessary for the portion of the tube surrounded by the heating member to be located as accurately as possible relative to said member in directions [transverse to the longitudinal direction of the tube and also necessary for this locating to remain unchanged during the displacement of the tube in its longitudinal direction.

It has been found that ditficulties may arise in this respect when using the above-mentioned known apparatus. Thus, it has been found difficult to obtain tubes of fire-proof material such as employed in these known apparatus, for example tubes of quartz glass, in an accurately rectilinear form and tubes of such materials which are commercially sold thus sometimes show very faint curvatures. Since the points of support for the tube, in this case the bearings, are comparatively far remote from the heating member in the known apparatus, a deviation in the rectilinear form of the tube will result in a comparatively great deviation from the desired location. Furthermore, in zone-melting comparatively refractory compounds, there is a risk of fiection of the tube.

An object of the invention is inter alia to obviate these disadvantages. According to the invention, the heating element itself is designed, at least in part, as a bearing for the tube.

Preferably, the heating member is made of graphite at least insofar as the bearing is concerned. Graphite is a refractory material which is particularly suitable for obtaining a low coefficient of friction even at high temperatures.

The heating member preferably comprises an electric-ally conductive ring and a high-frequency coil surrounding it, the ring being formed as a bearing for the tube. This ring is preferably made wholly of graphite. It is to be noted that the use of an electrically conductive ring, for example of graphite, and of a high-frequency coil surrounding it is known per se in zone-melting devices, but there the ring is then not for-med as a bearing for a tube passed through the ring.

Preferably, the heating ring is formed so that the central portion thereof, which is located inside the highfrequency coil, does not contact the tube whereas the adjacent portions of the ring are not surrounded by the high-frequency coil and the tube is journalled in the lastmentioned portions of the ring. The central portion has, for example, an internal diameter which is larger than that of the adjacent portions. Especially in zone-melting comparatively high-melting temperature materials, in which the central portion of the ring has to be heated to a high temperature, strong heating of the wall of the tube due to thermal conduction is thus prevented. The parts used as a bearing for the tube will likewise be heated due to transmission of heat from the heating ring and due to absorption of energy from the high-frequency field, so that they can also act as a radiation screen, thus obt aining a more progressive cooling of the crystallizing material upon displacing the molten zone through the charge to be treated. Furthermore the central portion has an external diameter which is preferably smaller than that of the adjacent portions. The central portion may thus have a comparatively small diameter, which counteracts undue dissipation of heat to the adjacent portions. However, the latter portions have a comparatively high thermal capacity so that fairly strong variations in temperature occur in the central portion, the temperature being highest at the areas midway between the two thicker parts and the temperature decreasings comparatively rapidly towards the thicker parts. A molten zone of companatively small length may thus be formed in the charge.

The heating ring is preferably arranged within a second tube which occupies a fixed position relative to the heating means, the ring being held in position by this second tube, for example due to the second tube having a diameter which is smaller on each side of the ring than around the ring. If the central portion of the ring is given an external diameter which is smaller than that of the adjacent portions, the second tube is also prevented from contacting with the hottest part of the ring.

In zone-melting compounds containing a volatile constituent in a closed vessel, or in zone-melting a material in the presence of the vapour of a doping substance, it is known to use a closed ampulla which contains both the material to be treated and the volatile constituent or the doping substance. By heating the whole of the ampulla, the desired vapour pressure of the volatile constituent or of the doping substance is built up. Known zone-melting devices for this purpose which have been suggested, for example, for zone-melting gallium arsenide in the presence of arsenic vapour, comprise for this purpose two ovens placed one behind the other with the usual high-frequency coil between them. The wall of the ampulla containing the charge to be treated, which ampuila is usually of a cylindrical shape, is heated by the ovens insofar as it is situated within them whereas the portion between the ovens has to be heated by the radiation originating from the molten zone. A difficulty then arises in that, before the molten zone is formed, the required vapour pressure has to be built up by heating by means of the ovens alone. The part of the wall situated between the two ovens then remains comparatively cold so that condensation may occur on it and the required vapour pressure can be obtained only after the molten zone has been formed. In addition, in the known device, a considerable thermal loss may occur due to radiation of the molten zone.

When using the device according to the invention which utilises an electrically conductive ring and a high-frequency coil surrounding it, the ring being formed as a bearing for the displaceable tube, it is possible to arrange around the tube two ovens, one on each side of the high-frequency coil, without the above-mentioned disadvantages occurring. In one preferred embodiment, to this end, the ovens are so arranged, and the ring is so proportioned, that the ring extends into the ovens. On building up the vapour pressure in the vessel, the high-frequency coil may be energized so that the ring is heated sufficiently to prevent condensation on the wall of the ampulla without the formation of a molten zone. If the ring has a central portion within the high-frequency coil which does not contact the tube and two adjacent portions which serve as bearings for the tube, the central portion preferably lies wholly outside the ovens.

To obtain a smooth movement of the displaceable tube relative to the heating member, the tube is preferably moved in its axial direction by means of rollers. To this end, the tube is preferably provided with a metallic sleeve which can move between the rollers.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing showing a vertical section of a zone-melting device.

The sole figure shows in broken line a sealed ampulla 1 with a vessel 2 in it containing a rod 3 of semi-conductor material, a movable tube 4, a graphite ring comprising a thin central portion 6 between two thicker portions 5, in which ring the tube 4 is journalled, a highfrequency coil 7 which is connected to a high-frequency generator (not shown), a fixed tube 8 having a closure 9 and an insulating diaphragm 10, two heating ovens 11 and 12, a sleeve 13 provided at the outer end of the movable tube, ball-bearings 14 for a free displacement of the sleeve 13 in the direction of its length, a roller 15 to be driven by an electric motor (not shown) for driving the sleeve 13 and the tube 4 (by friction), a nozzle 16 for the supply of gas and a nozzle 17 for the discharge of gas.

For certain semi-conductors which do not evolve gas or vapour and which may be treated in an inert atmosphere, the closed ampulla 1 may be dispensed with. This atmosphere may then consist, for example, of a continuous flow of nitrogen or other indifferent mixture of gasses which is supplied through the nozzle 16 and discharged through the nozzle 17. When using the ampulla 1, an oxygen free gas is also supplied through the tubes to prevent the graphite ring from being burnt.

The assembly comprising the movable tube 4, the vessel 2, the semi-conductor rod 3 and as the case may be, the ampulla 1 is given a uniform rectilinear movement in the direction indicated by arrow 18, or in the opposite direction, indicated by a broken arrow, due to the action of the driving roller 15 on the metal sleeve 13. The sleeve 13 is preferably made of stainless steel and its outer surface extends parallel with the axis of the tube 4 in order that its displacement on the rollers 14 (which, as previously mentioned form ball-bearings) takes place freely and without any appreciably irregularity.

The displacement of the sleeve 13 naturally controls that of the tube 4, which is integral with it, and of the assembly inside the tube. In the device shown in the figure, the relative movement of the semi-conductor body and the heating means no longer depends upon a metal cable, as is the case in known devices and apparatus, but on the sleeve 13 which operates as an independent pulling mechanism. The movable tube 4 is passed hermetically and in a slidable manner through the diaphragm 10 which closes that extremity of the fixed tube 8 through which the tube 4 emerges from the tube 8. Said diaphragm is made of a heat-proof and gastight insulating material which permits a gas-tight passage with small friction of the movable tube. It is possible, for example, to use polytetrafiuoroethene for said diaphragm.

The fixed tube s preferably comprises three portions, namely, a central portion located between two lateral portions 8a having an internal diameter which is a little smaller than that of the central portion. Under these conditions, the junctions between the central portion and the lateral portions form locking rings for the graphite ring the outer portions 5 of which serve as bearings for the tube 4 and the inner portion 6 of which serves to ensure a uniform axial distribution of temperature, as a short-circuiting secondary for the high-frequency coil '7 whereby it is heated and radiates heat so that a molten zone is formed in the rod 3, and as a spacer for the two outer portions of the ring. Due to this arrangement, the graphite ring is substantially held in position within the tube 23. The thin central portion 6 of the graphite ring entirely lies outside the ovens 11 and 12, whereas its outer portions 5 are substantially located inside the ovens. On displacement of the movable tube, the molten zone produced is moved through the tube 4 in a manner known per se. The material crystallizing behind it is now progressively cooled to the temperature of the oven 12 as the tube 4 moves in the direction indicated by arrow 18, since the heating ring, as reckoned from the central portion 6 to the two ends of the ring, shows a gradual decrease in temperature. For obtaining a view of the molten zone, the portion 6 of the ring may be provided with one or more apertures which must, however, be small to prevent an appreciable local cooling of the molten zone due to radiation. The device shown need not necessarily be arranged so that the vessel 2 is exactly in a horizontal position. If desired, the apparatus may be positioned so that the vessel is at a small angle to the horizontal line.

Although a device for horizontal zone-melting has been described by way of example, the invention is not limited thereto and also relates to devices for floating zone-melting of a rod positioned vertically. In addition, the device is applicable to purifying, doping, as Well as formation of monocrystals by zone-melting.

It will be evident that numerous embodiments are possible within the scope of the invention.

What is'claimed is:

l. Zone-melting apparatus comprising an elongated tubular member for supporting an elongated charge of a material to be treated by zone-melting, high-frequency heating means for said charge, said high-frequency heating means comprising an electrically-conductive ring surrounding a section of the tubular member and a highfrequency heating coil surrounding a peripheral portion of said ring, whereby when said high-frequency currents traverse the heating coil, heat is induced in the ring which in turn melts a transverse zone of this charge lying within it, said electrically-conductive ring including a tubular portion which contacts and bears against substantially all sides of a peripheral section of the tubular member whereby said tubular member is journalled in the said electrically-conductive ring, which thus serves as a bearing for the tubular member, and means for moving the tubular member and the electrically-conductive ring relative to one another in the longitudinal direction of the tubular member for advancing the molten zone through the charge.

2. Apparatus as set forth in claim 1 wherein the tubular bearing portion of the electrically-conductive ring is wholly graphite.

3. Zone-melting apparatus comprising an elongated tubular member for supporting an elongated charge of a material to be treated by zone-melting, high-frequency heating means for said charge, said high-frequency heating means comprising an electrically-conductive ring surrounding a section of the tubular member and a highfrequency heating coil surrounding a peripheral portion of said ring, whereby when high-frequency currents traverse the heating coil, heat is induced in the ring which in turn melts a transverse zone of the charge lying areaevs within it, said electrically-conductive ring comprising a central portion lying within the high-frequency coil but spaced from the tubular member and adjacent outer portions lying outside the high-frequency coil but contacting a pheripheral section of the tubular member whereby said tubular member is journalled in the said outer portions of the electrically-conductive ring, which thus serve as bearings for the tubular member, and means for moving the tubular member and the electrically-conductive ring relative to one another in the longitudinal direction of the tubular member for advancing the molten zone through the charge.

"4.- Apparatus as set forth in claim 3 wherein the entire electrically-conductive ring is of graphite.

5. Apparatus as set forth in claim 3 wherein the inside diameter of the said central portion is larger than that of the said outer portions, and the outside diameter of said central portion is smaller than that of the said outer portions.

6. Apparatus as set forth in claim 3 wherein a second tubular member surrounds the electrically-conductive ring inside the heating coil, said second tubular member being provided with means for gripping and fixing the position of the said electrically-conductive ring.

7. Zone-melting apparatus comprising an elongated, insulating, tubular member for supporting an elongated charge of a material to be treated by zone-melting, highfrequency heating means for said charge, said highfrequency heating means comprising an electrically-conductive ring surrounding a section of the tubular member and a high-frequency heating coil surrounding a peripheral portion of said ring, whereby when high-frequency currents traverse the heating coil, heat is induced in the ring which in turn melts a transverse zone of the charge lying within it, said electrically-conductive ring comprising a central portion lying within the high-frequency coil but spaced from the tubular member and adjacent outer portions lying outside the high-frequency coil but contacting a peripheral section of the tubular member whereby said tubular member is journalled in the said outer portions of the electrically-conductive ring, which thus serve as bearings for the tubular member, a pair of furnaces arranged around the said tubular member on opposite sides of the heating coil, the outer portions of said electrically-conductive ring each extending within one of the furnaces, and means for moving the tubular member and the electrically-conductive ring relative to one another in the longitudinal direction of the tubular member for advancing the molten zone through the charge.

Apparatus as set forth in claim 7 wherein the central portion of the electrically-conductive ring is located wholly outside of the said furnaces.

9. Apparatus as set forth in claim 7 wherein a roller is arranged in contact with said tubular member, and means are provided for rotating the roller to move the tubular member through the heating means.

References Cited by the Examiner UNITED STATES PATENTS 2,826,666 3/58 Cater 23-273 X 2,898,249 3/59 Jensen 23-301 X 2,902,350 9/59 Jenny et al 23-301 X RICHARD M. WOGD, Primary Examiner. JOSEPH V. TRUHE, Examiner. 

1. ZONE-MELTING APPARATUS COMPRISING AN ELONGATED TUBULAR MEMBER FOR SUPPORTING AN ELONGATED CHARGE OF A MATERIAL TO BE TREATED BY ZONE-MELTING, HIGH-FREQUENCY HEATING MEANS FOR SAID CHARGE, SAID HIGH-FREQUENCY HEATING MEANS COMPRISING AN ELECTRICALLY-CONDUCTIVE RING SURROUNDING A SECTION OF THE TUBULAR MEMBER AND A HIGHFREQUENCY HEATING COIL SURROUNDING A PERIPHERAL PORTION OF SAID RING, WHEREBY WHEN SAID HIGH-FREQUENCY CURRENTS TRAVERSE THE HEATING COIL, HEAT IS INDUCED IN THE RING WHICH IN TURN MELTS A TRANSVERSE ZONE OF THIS CHARGE LYING WITHIN IT, SAID ELECTRICALLY-CONDUCTIVE RING INCLUDING A TUBULAR PORTION WHICH CONTACTS AND BEARS AGAINST SUBSTANTIALLY ALL SIDES OF A PERIPHERAL SECTION OF THE TUBULAR MEMBER WHEREBY SAID TUBULAR MEMBER IS JOURNALLED IN THE SAID ELECTRICALLY-CONDUCTIVE RING, WHICH THUS SERVES AS A BEARING FOR THE TUBULAR MEMBER, AND MEANS FOR MOVING THE TUBULAR MEMBER AND THE ELECTRICALLY-CONDUCTIVE RING RELATIVE TO ONE ANOTHER IN THE LONGITUDINAL DIRECTION OF THE TUBULAR MEMBER FOR ADVANCING THE MOLTEN ZONE THROUGH THE CHARGE. 